(ITS#7073) zero-length entries should be allowed

Full_Name: Howard Chu
Version: 2.4
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (76.94.188.8)
Submitted by: hyc
entry_header() rejects "entries" that have zero attributes. As it turns out,
overlays like pcache and translucent can create such entries in their normal
operation. With back-bdb/hdb, such entries will work OK while slapd is running,
because they're in the entry cache. But after a restart, such entries can't be
read back from the DB because they're considered invalid by entry_header().
Already fixed in master, entry_header() now allows zero length entries.